The present invention relates to a method of adjusting capacities of combustion chambers of a multi-cylinder engine, and particularly relates to a method of adjusting capacities of combustion chambers of a multi-cylinder engine, in which the combustion chamber capacities of a cylinder head formed by casting are adjusted by using cutting margins of surfaces of the combustion chambers.
Conventionally, because a theoretical thermal efficiency of a spark-ignition gasoline engine increases in proportion to a compression ratio up to about 17:1, the increase of the compression ratio is proposed as an effective means for improving fuel consumption. Further, a pent-roof combustion chamber in which an intake port side inclined surface and an exhaust port side inclined surface are formed in a roof shape is known as a combustion chamber structure effective for realizing a high compression ratio. The pent-roof combustion chamber is considered to be an effective structure because a comparatively large diameter can be secured for intake and exhaust valves, a combustion chamber capacity can be made smaller in comparison to a cylinder capacity, and it is convenient in controlling a flow, such as a tumble flow (vertical whirling flow inside the cylinder) or a swirling flow (horizontal whirling flow inside the cylinder) of intake air inside the cylinder.
A cylinder head of a spark-ignition gasoline engine disclosed in JP H9-119344A includes, in a single cylinder, two intake ports and two exhaust ports, an ignition plug hole, and a cooling water passage. A part of the cooling water passage that will be located below the intake ports is formed by a metal casting mold serving as a lower surface main mold of the cylinder head, and a part of the cooling water passage that will not be below the intake ports is formed by a sand core. These parts of the cooling water passage are manufactured to communicate with each other via a mechanical fabrication operation after casting. In such a cylinder head including pent-roof combustion chambers, the part of the cooling water passage that will be arranged below the intake ports is formed by a metal casting mold, therefore the casting quality can be raised by preventing a sand core for forming the cooling water passage that will be arranged below the intake ports from being damaged, and the anti-knocking ability can be improved by precisely controlling the dimensions and thus the capacity of the cooling water passage.
Theoretically, a compression ratio ε can be calculated from the following equation.ε=((π/4)×b2×s+V)/V 
In the equation, “b” indicates a bore diameter of the cylinder, “s” indicates a stroke length of a piston, and “V” indicates the combustion chamber capacity. When increasing the compression ratio, an abnormal combustion, such as knocking, pre-ignition, or detonation, easily occurs. Therefore, combustion is normally controlled to suppress abnormal combustion by delaying an ignition timing by a predetermined time period according to a preset compression ratio from the point when the piston reaches the top dead center in the compression stroke.
A method of metal mold casting where a molten metal is filled into a molding cavity is utilized in manufacturing the cylinder head, and a method of low pressure casting utilizing a comparatively low pressure (e.g., about 0.5 kg/cm2 or below) is often adopted in manufacturing a casting product made of light alloy such as an aluminum alloy. The molten metal filled into the molding cavity is solidified after a predetermined time has elapsed to form a cylinder head material. Then, the cylinder head material is demolded from the metal casting mold, and, as a post process, the demolded cylinder head is cut to form, for example, a mating surface for mating with a cylinder block. For this reason, even if the molding cavity is precisely formed for producing the combustion chambers with a target capacity within the metal casting mold, the cooling characteristics of the molten metal may differ among the cylinders and a shrinkage difference may occur in the combustion chambers of the cylinders due to, for example, the solidification characteristics.
If the shrinkage difference due to the solidification characteristics occurs in the combustion chambers of the cylinder head material, the combustion chamber capacities of the cylinders vary, thereby a uniform combustion among the cylinders cannot be obtained and the combustion status of the engine may negatively be affected. Further, in a high compression ratio engine, if a balance among a predetermined fuel supply amount, the compression ratio, and the ignition timing is lost, the abnormal combustion as described above may occur.